Phosphonate-functionalized magnetic mesoporous silica for rare earth element separation from citric acid extraction media

D. Li, Y. Tang, L. Hu
Savannah River National Laboratory,
United States

Keywords: mesoporous silica, phosphonate functionalization, rare earth element separation, citric acid extraction


Rare earth elements (REEs) are critical materials that are widely used in various industries and innovative technologies. The US relies heavily on international supplies of REEs and may be at high risk of supply disruption due to potential geopolitical conflicts. Thus, it is critical for the US to seek new alternative and sustainable technologies for domestic REE production. Electronic and industrial wastes contain reasonably high REE content and may offer new opportunities for REE recovery. Our research aims to design and investigate molecular mesoporous materials for effective and selective REE extraction/separation from mild acidic extract of electronic and industrial wastes, such as municipal solid waste incineration ash (MSWIA) and magnet scrap. Specifically for this reported work, we developed phosphonate-functionalized magnetic mesoporous silica (MMS) for REE separation from citric acid media that aimed to simulate the greener and less hazardous extraction of MSWIA. MMS material was synthesized using cetyltrimethylammonium bromide template in the presence of magnetite nanoparticles. The MMS material was functionalized with phosphonate ligand (MMS-PP) using a post-synthesis method. Both MMS and MMS-PP materials were characterized by powder XRD, small angle XRD, SEM-EDS, TEM, FTIR, solid state NMR spectroscopy, and N2 adsorption-desorption isotherm measurement. These results demonstrated that the MMS-PP was organic-inorganic hybrid material with a magnetite core for easy retrieve, a mesoporous silica matrix (3.3 nm pore size) for high surface area and multiple active sites, and surface functional phosphonate ligand for improved capacity and selectivity toward REEs over other competing cations. Both MMS and MMS-PP materials were evaluated for four highest REEs in MSWIA, lanthanum (La), cerium, neodymium, yttrium separation, from simulants with incrementally complex chemistries. Both MMS and MMS-PP materials were equally effective at La separation (>99%) from 69.5 ppm La DI water (pH ~6.0, sorbent/liquid ratio 10 g/L). In 50 mM citric acid (pH ~2.1), the La recovery rates of MMS decreased to nearly zero, but the La recovery rate of MMS-PP remained at 91%. With different La loadings in DI water and 50 mM citric acid solution, the La recovery rate of both MMS and MMS-PP decreased with increasing La loadings. However, MMS-PP exhibited much higher La recovery rate at the identical La loadings in the same solution: >96% at the La loading of 417 ppm in DI water, and >60% at the La loading of ~200 ppm in the 50 mM citric acid simulant. The MMS-PP material exhibited excellent selectivity for La separation against competing Na+ and Ca2+, but its selectivity against Al3+ and Fe3+ was moderate. In addition, the adsorption reaction of La onto MMS-PP was fast, completing within 10 minutes, and the MMS-PP materials remained effective at La separation after three adsorption-desorption cycles. We will continue combining rational material design, theoretical computation and machine learning, advanced molecular-level characterization, and robust engineering processes to develop advanced porous materials and provide a green, effective, tunable, and potentially universal solid-liquid separation platform for REE and other critical metal extraction and recovery from complex electronic and industrial wastes, as well as mineral ores.